Lathyrus odoratus, commonly known as the sweet pea, is a highly favored annual flower celebrated for its delicate, ruffled blossoms and intense fragrance. Gardeners who wish to enjoy these blooms for the longest possible season must understand their preference for cool weather. Success with this popular vining plant largely depends on precise timing, especially when moving seedlings that were started indoors. Transplanting sweet peas at the correct stage and under the right environmental conditions ensures they establish a robust root system before the onset of summer heat.
How to Determine the Best Time to Transplant
The most important factor for timing the transplant is the last expected frost date. Sweet pea seedlings are notably cold-tolerant and thrive when planted four to six weeks before this date, allowing them to establish roots in cool soil. This early planting ensures they can complete their growth cycle before temperatures become too high.
In regions with prolonged cold winters, transplanting should occur as soon as the soil is workable in late winter or very early spring, even if there is still a risk of light frost. Sweet peas can generally withstand temperature dips as low as 20°F for short periods. Gardeners in mild-winter climates, such as USDA Zones 8 through 10, often plant their sweet peas in late fall to establish them for an early spring bloom.
Sweet peas are ready to be moved from their indoor containers to the garden bed when they are approximately 3 to 4 inches tall. At this size, they should have developed three to four sets of true leaves, indicating they have enough foliage to begin photosynthesis outdoors. Placing the young plant into its final position while the weather is still cool is necessary because the sweet pea naturally declines once the temperature consistently rises above 80°F.
The Critical Step of Preparing Seedlings
A process called “hardening off” must be completed to prepare the plants for outdoor life. This step is the gradual acclimation of the indoor-grown seedlings to the harsher conditions of direct sunlight, fluctuating temperatures, and wind. Skipping this phase can result in “sun scorch,” where the tender leaves turn white and die back.
The hardening off process should span a period of seven to fourteen days. Begin by placing the seedlings in a sheltered, shaded outdoor location for one to two hours during the morning. Over the course of the next week, the time they spend outdoors should be increased by an hour or two each day.
Gradually introduce the young plants to more direct sunlight and increased exposure to wind. It is necessary to bring the seedlings back indoors or into a protected cold frame every night, especially if nighttime temperatures are predicted to drop below freezing.
Pinching the Growing Tip
Pinching the central growing tip encourages a bushier, more productive plant. When the seedlings reach about 4 to 6 inches in height, use a clean snip or your fingers to remove the growing tip just above a set of leaves. Removing this apical dominance stimulates the plant to produce side shoots from lower leaf nodes, which ultimately results in a greater number of flowering stems. This pinching can be done during the hardening-off period.
Successful Transplanting and Initial Support
The location chosen for transplanting should have well-drained soil, ideally with a pH between 6.0 and 7.0. Sweet peas are known to be heavy feeders, so the soil should be amended with organic matter like compost or well-rotted manure prior to planting.
Sweet peas are highly sensitive to root disturbance, meaning the physical act of moving them requires gentle handling to prevent transplant shock. It is best to start the seeds in deep containers, such as root trainers, and plant them directly into the ground without disturbing the root ball. When planting, set the seedling into the prepared soil and gently firm the soil around the base, spacing the plants approximately 4 to 8 inches apart.
A thorough watering should immediately follow planting to settle the soil around the roots. Sweet peas are vining plants that can quickly reach heights of six to ten feet, so a support structure must be established simultaneously with the transplant. Providing a trellis, netting, or stakes right away allows the young plant’s tendrils to immediately grasp the support. Allowing the vines to sprawl on the ground, even briefly, can result in tangled and less vigorous growth, compromising the plant’s overall health and flower production.